The sedimentary record of the exhumation of a granitic intrusion into a collisional setting: The lower Gonfolite Group, Southern Alps, Italy

The clastic wedge of the Gonfolite Lombarda Group (GLW) accumulated during Oligocene-Miocene times in the Southern Alps foreland basin, formed on the southern, inner side of the Alpine belt. It represents the depositional counterpart of the exhumation and erosion of the Central Alps metamorphic-magmatic units. Among the Central Alps units, the Tertiary Bergell Intrusion (TBI) is one of the principal sources of pebbles occurring within the GLW. Geochronologic data, both from intrusive pebbles and present-day outcrops of intrusive rocks, document the rapid uplift history of the GLW source area. The lower Gonfolite clastic wedge (Como Conglomerate and Val Grande Sandstone Formations, Oligocene-Early Miocene) has been investigated through the study of sandstone and conglomerate petrology for detecting the effects in the sedimentary record of this collision-related event. The main results are: (i) sandstone petrology of the Como Conglomerate records an evolution from feldspatholithic to feldspathic sandstones; (ii) the related Q/F-F/L ratios suggest an evolution from a mixed plutonic-metamorphic to a mainly plutonic source; (iii) consistently, conglomerate petrology records a progressive increase of plutonic pebbles (from nearly 0-50% of the total), a corresponding decrease of metamorphic clasts (from nearly 80 to nearly 50%) and the disappearance of cover rock fragments. Considering the high relief/short transport setting of the GLW clastic routing system, these values probably resemble the real proportions of such rocks in the Gonfolite catchment area. During the Aquitanian, the return to a metamorphic-rich source is recorded both by sandstones and conglomerates at the top of the Como Conglomerate and in the Val Grande Sandstone. This last signal is interpreted as the result of the reorganisation of the Gonfolite source area, possibly related to the northward shift of the main Alpine divide.